<!DOCTYPE html>
<html lang="zh-CN">
<head>
    <meta charset="UTF-8">
    <title>12_margin的注意事项</title>
    <style>
        box {
            width: 400px;
            height: 400px ;
            padding: 50px;
            background-color: aqua;
        }
    </style>
</head>
<body>
    <div box="box1">1</div>
    <div box="box2">2</div>
    <div box="box3">3</div>
</body>
</html>